1. Field of the Invention
The present invention relates to carbonized plants, and more particularly relates to the fabrication of materials using wood and other naturally fibrous plants as precursor materials. The carbonization process retains the anatomical features of the precursor plant while converting the composition of the plant to primarily carbon. The carbonized wood may then be formed to the desired shape. The shaped carbon product may be used to form composites such as carbon--carbon and carbon-polymer composites. The shaped carbon product may alternatively be converted to ceramic compositions, or further processed to form ceramic-containing composites such as ceramic-metal and ceramic--ceramic composites.
2. Background Information
Carbonization of wood for the manufacture of charcoal has been practiced since the beginning of history. Destructive distillation was practiced by the ancient Chinese. The Egyptians, Greeks and Romans carbonized wood and distilled the volatiles for embalming purposes and the filling of joints in wooden ships. In ancient times wood charcoal was used for the removal of odors, medicinal purposes, domestic cooking fuel, the making of gunpowder and the refining of ores. The Industrial Revolution brought about a heavy demand for charcoal, especially for the making of iron. Up until the late 1800's the largest portion of manufactured wood charcoal went into the reduction of iron ores. Today, coal derived cokes are used.
For many centuries charcoal was made in open air pits in the Western world. This entailed tightly piling bolts of air dried wood end to end forming a conical mound. This mound was then covered with several inches of leaves, grass, needles, branches or moss depending upon what was available. An additional few inches of dirt or sod then capped off the mound. Openings were left at the base for air supply and up the center to allow smoke to escape. The mound was then ignited at the base through an opening. The mound tender made certain just enough air entered to allow a smoldering combustion which could take from one to several weeks to complete.
The production of wood charcoal became a major industry by the end of the nineteenth century. The conventional open pit methods wasted the by-product gasses that are released when wood is thermally decomposed. An entire industry was formed around the distillation of the vapors evolved from wood carbonization. In the U.S. two branches of the industry formed due to the fact that denser hardwoods give different products than the lighter, more resinous softwoods. The products from destructive distillation of hardwoods included wood alcohol (methanol), acetate of lime and charcoal. The softwoods gave turpentine, tar, wood oils and charcoal. These products were made possible by carbonizing wood in a container designed such that the evolved gasses could be captured and distilled.
The first development beyond the open pit method was the use of brick kilns designed to both contain the wood charge and provide a means to tap into the exhaust. The brick kiln method meant a faster production rate since mounds were not needed and rapid loading was possible. This was also an asset for the iron industry which was rapidly expanding. The heat needed for decomposition was obtained from the wood charge itself, just as in the open pit method. One drawback of the brick kiln was that a large portion of the evolved vapors were lost through the bricks, thus giving limited yields.
The first high efficiency device for collecting carbonized wood vapors was the small cylindrical retort made of cast iron or steel measuring some 4 ft in diameter and 9 ft in length, capable of holding about two thirds of a cord. These were installed horizontally as pairs with batteries of 10 or more pairs in long rows enclosed by brick. Heating was able to be provided externally from below the retorts and fuel was typically in the form of charcoal, coal, wood gas, wood oil, wood tar or wood alone. A single run took about twenty four hours to complete. The vapors were collected and distilled in the form of pyroligneous acid which was later refined to produce acetic acid, methanol, acetone, furfural, tars and oils.
The cylindrical retort evolved into a large rectangular steel retort enabling the use of cars for loading and unloading. A common size was 50 ft long by 8 ft high and 6 ft wide. These retorts held more than ten cords of wood and considerably increased production rates while reducing the amount of labor involved. After a twenty four hour carbonization cycle the cars were removed to cooling retorts and held for another one or two days. Once removed from a cooling retort the cars were allowed to sit in the open for another two days thus giving a total time, from wood to marketable charcoal, of about ninety six hours. Some of the larger wood carbonization and distillation plants consumed as much as 200 cords per day.
In the beginning of the twentieth century wood charcoal and its distillation products had fallen behind the products derived from coal and petroleum in several of the markets. Eventually, due to dwindling supplies of wood and the availability of higher grades of coals, the metallurgical market share became dominated by coal derived cokes. The demand for wood charcoal began to decline in the late 1800's. Petroleum based products also began to take over some of the markets dominated by wood distillation products.
Currently in the U.S., the only significant markets for wood carbonization products are activated carbons and charcoal briquettes.
Thermal degradation of wood has been studied with the intent of gaining information on its ignition characteristics. In the U.S. wood is used on a large scale in construction, especially for residential housing. Increased public concern over fire safety has prompted forest product industries to investigate methods for eliminating or reducing the ignition temperatures of wood and wood based products. The most common approach was chemical treatment to suppress various decomposition reactions. Progress in many cases was limited due to the potential for production of poisonous, or noxious fumes when products did finally ignite. Additional problems such as degradation of wood mechanical properties and bonding characteristics has hindered wide scale manufacture of fire retardant products. In the 1980's construction grade fire resistant plywood entered the market and was widely installed as roof sheathing in many regions of the U.S. Unfortunately, the release to market was premature as the product suffered from ply delamination resulting in product recall and much negative publicity.
Several U.S. patents address wood treatment methods. U.S. Pat. No. 1,237,521 to Jennison discloses impregnating wood with preservatives such as tar.
U.S. Pat. No. 1,483,733 to Kozelek discloses the production of wood for musical instruments by heating the wood in air to a temperature of from 450 to 550.degree. F. (232 to 288.degree. C.). The heat treated wood, which has a yellow color, may then be treated with varnish prior to making the musical instrument.
U.S. Pat. No. 3,508,872 to Stuetz et al. discloses a process for the production of graphite fibrils using wood splinters less than 0.5 inch in length as a starting material. The splinters are first heated in air at 150 to 400.degree. C., and are then charred at 2000 to 3000.degree. C. The resulting graphite fibrils are then incorporated in a binder to form a composite.
U.S. Pat. No. 3,927,157 to Vasterling discloses the production of carbon--carbon composites using wood pulp as a starting material. Carbohydrate sugars are first chemically extracted from the wood, followed by heating the wood pulp at increasing temperatures of up to at least 3800.degree. F. The fibers are then mixed with a carbonizable binder and the mixture is heated to form the carbon--carbon composite.
U.S. Pat. No. 4,170,668 to Lee et al. discloses a method for pre-charring the surface of wood in order to retard fire and rot.
U.S. Pat. No. 4,678,715 to Giebeler et al. discloses the impregnation of wood with thermosetting polymers.
U.S. Pat. No. 5,143,748 to Ishikawa et al. discloses the surface treatment of wood with a plasma to impart water repellency.
The manufacture of graphite products is well known. Molded graphites are conventionally produced by a compaction process using a mixture of carbon filler with an organic binder which is heat treated to produce parts such as large electrodes used in metallurgical processes. In the late 1800's, E. G. Acheson patented a process for manufacturing molded graphite parts which uses an electric resistance furnace for heat treatment of green products at temperatures adequate for graphitization to occur (about 3000.degree. C.). This was a significant development as it enabled carbon electrodes with relatively low resistivity to be produced. Many improvements have since been made and the applications for molded graphites has increased significantly since then. In the 1940's, Enrico Fermi first used molded graphite as a moderator for a self sustaining nuclear reaction. Other modern applications of molded graphites include use as a refractory, electric motor brushes, electrical resistance heating elements in high temperature furnaces, rocket nose cones, rocket exit cones and various other aerospace components.
Unlike molded graphites, glass-like carbons do not readily graphitize and exhibit isotropic properties. Glass-like carbons are used as vessels in chemical processing or analytical chemistry. They are also used as crucible material for the melting of noble metals and special alloys, especially in dental technology. Glass-like carbons in the form of small spheres are being considered for uses as catalyst supports. In addition, glass-like carbons are being produced in the form of open-cell foams.
Carbon foams are a fairly recent addition to the family of solid carbon materials. These are glass-like carbons produced in the form of an open pore foam. They are reported as having potential applications including catalyst supports, battery anodes, micro-porous membranes for filtration, supercapacitor electrodes, low mass structural materials and composites.
There are several reported processes and precursors used to produce carbon foams, also referred to as reticulated carbons. A polymer which is highly cross-linked and does not go through a fluid state is the first criteria for selecting a precursor. Some of the polymers of choice are furfuryl alcohol, phenolics, polyacrylonitrile, polyurethane, resorcinol and others. In one process an inorganic is removed by leaching after carbonization, leaving a replica carbon which is then freeze-dried. Other carbon foams are produced by the blowing of bubbles into a variety of liquid polymers.
Pyrolytic carbons and pyrolytic graphites are different in that they are produced by chemical vapor deposition (CVD) from organic vapors. They also differ from other forms of solid carbons in that the main application is as a coating. Pyrolytic graphite was first produced in the late nineteenth century and was used for lamp filaments. Although produced as a coating, it can be made thick enough such that after removal from the substrate it has sufficient mechanical integrity to stay together. Some applications of these films, which vary in degree of crystallographic order, are heart valves and dental implants, coatings on molded graphite parts, coatings on fibers--especially ceramic fibers which are reactive with their composite matrix, coatings on optical fibers for improved abrasion resistance and infiltration coating of carbon fiber preforms for the manufacture of carbon--carbon composites (also termed chemical vapor infiltration, CVI).
Carbon fibers have been produced using polyacrylonitrile (PAN) as a precursor. Fibers of ultrahigh modulus having a modulus of elasticity greater than 50% (&gt;500 GPa) of the value of C.sub.11 for graphite single crystals have been made.
During the 1970's progress was made in the use of pitch as a less expensive carbon fiber precursor. These precursors are capable of producing carbon fibers of ultra high modulus but, in general, of lower strength than those derived from PAN. The main difference between carbon fibers derived from pitch and those from PAN lies in the degree of crystallization and structural morphology of the solid carbon fibers. In general, PAN derived carbon fibers are non-graphitic while pitch derived carbon fibers are graphitic. While some present day applications utilize pitch based carbon fibers the majority of the market is taken by carbon fibers derived from PAN.
Carbon black and lampblack are forms of solid carbons produced by thermal decomposition of organics resulting in the formation of solid particles in the gas phase. Their difference lies primarily in the organic precursor and the size and atomic structure of the resulting solid carbons. Lampblack is produced from the burning of oils, tars or resins in an oxygen limited environment. Carbon black is manufactured by incomplete combustion of a gas. Lampblack is one of the oldest forms of manufactured carbon, and the first known commercial process for making nano-particles. It was made by collecting the smoke from an oil lamp. Lampblack is still used today as a black pigment in inks and paints.
Carbon black is produced by the channel process or the thermal process. In the channel process, small flames of natural gas impinge upon a cool metal surface in the form of a channel, a rotating disk or a roller. The carbon powder forms on the cool surface and is then exposed to a high temperature to oxidize the particle surface. The thermal process, also termed "cracking", produces carbon black by thermal decomposition of natural gas in the oxygen free environment of a preheated chamber. Acetylene black is a special type of carbon black which is derived from the thermal decomposition of acetylene. Carbon black is used commercially in large quantities for the reinforcement of rubber in the tire industry.
Activated carbons are processed solid carbons with a highly developed porous structure and large internal specific surface area (&gt;1000 m.sup.2 /g). These processed solid carbons, which were developed as improved adsorbents for the decolorization of sugar, can be produced by heat treatment in the presence of steam or carbon dioxide. An alternate method for producing activated carbon is to impregnate various vegetable matter with salts prior to carbonization. These processes are still used today, with some modifications, for the production of activated carbons from a diverse group of organic precursors.
It is known that the presence of certain metals or metallic compounds during heat treatment of a non-graphitic carbon can cause graphitization to occur at temperatures well below what is otherwise required. It has also been established that non-graphitizable carbons may be transformed into graphitic carbons by heat treatment with additions of metallic compounds. This phenomenon has been given the name catalyzed graphitization.
Solid carbon as a structural material finds many applications. Many of these applications make use of the refractory properties of solid carbons. The combination of thermal stability, thermal shock resistance and high strength and stiffness at very high temperatures make solid carbon materials unique. One major disadvantage is their sensitivity to high temperature oxidation.
Monolithic carbon or carbon particulate composites are not reliably used in structural applications due to their brittle mechanical behavior, flaw sensitivity, variable properties and difficulties in fabrication of complex large components. Carbon fiber reinforced carbon matrix composites have been developed to overcome some of those limitations. Commonly referred to as carbon--carbon composites, these fiber reinforced materials are now being used in some of the most severe environments.
Applications for carbon--carbon composites include rocket nozzles, rocket reentry heat shields, shuttle nose cone, brake pads and rotors in aircraft and race cars. Other applications include refractory molds and dies, high temperature engines, corrosion resistant structural materials, heat exchanger tubes and biomaterials. Many applications utilize a three-dimensional reinforcement to achieve the properties desired.
The properties of solid carbons with identical composition can vary considerably. The reinforcing phase is generally required to have high stiffness and strength. Conventional PAN based fibers meet these requirements and are predominately used for both carbon-polymer and carbon--carbon composites.
The matrix carbon phase is typically derived from the carbonization of cross-linked polymers such as phenolics. These produce high carbon yields and a carbon phase which has distinctly different properties from the reinforcing fibers. The fiber-matrix bond is a critical factor in determining the mechanical properties of the composite. If the bond is too strong, crack deflection and fiber pull-out do not occur and the material exhibits little toughness.
Processing of carbon--carbon composites is typically accomplished by two different methods, polymer impregnation followed by carbonization, or chemical vapor infiltration (CVI). CVI entails the use of a carbonaceous gas, such as methane, which is allowed to infiltrate a heated carbon fiber preform where it decomposes, leaving a carbon residue on fiber surfaces. Its disadvantage is that often pores become choked off leaving closed porosity in the final composite. It also greatly increases manufacturing costs. Most commercial processes use polymer impregnation/carbonization.
Preforms of woven carbon fibers are conventionally impregnated with polymers by resin transfer molding techniques. Phenolics in an organic solvent can fully penetrate the carbon fiber weave. After solvent evaporation, phenolic carbonization is carried out. Yields of 60% are typically obtained from phenolics. The first carbonization step results in a composite with considerable porosity and a second, and possibly third, impregnation/carbonization sequence is necessary. Carbonization may be performed in a mold to limit distortion of the preform.
Another method for producing carbon--carbon composites is to start with a pre-impregnated carbon fiber weave and form the desired shape. Carbonization, and additional impregnation/carbonization steps are then performed. Other polymers, pitch for example, are used as precursors to achieve different matrix properties.
Fiber reinforced polymer composites have found widespread use in recent years. The largest market, by volume, is in the fabrication of boat hulls. This market is dominated by glass fiber composites. Carbon fiber composites are becoming less expensive and more competitive in some applications. Much of the development of carbon reinforced polymers (CRP) has been supported by the aerospace and aviation industries where specific stiffness and specific strength are critical issues. This development has led to diverse use in applications ranging from sports equipment to advanced aircraft components.
One of the advantages inherent in use of CRP's is that structures can be designed with a material tailored to the particular demands of the application. Composite properties can be made with varying degrees of anisotropy so an exact fit of material property to structure can be made. Complex shapes are also possible using composites since the structure is formed at the same time the material is made. This is made possible by use of several manufacturing approaches.
Manufacturing with carbon fiber composites is conventionally done by stacking woven or continuous fiber pre-pregs, by infiltrating mats or weaves with resins, by extrusion of chopped fibers mixed with polymers and by spraying fibers and resin into molds. Techniques have been refined such that composites of very high quality are becoming common in high performance applications. However, high performance composites are expensive, partially as a result of the costs incurred in forming a final product from the constituent materials. Stacking of pre-pregs is often done by hand thus adding substantially to final product costs. Polymer infiltration often causes fiber swimming and shifting of weaves. Infiltration often results in considerable porosity in the final product. These defects in composites severely limits their structural worth in safety critical applications.
Ceramic materials are often processed by complex procedures to attain a material with properties specific to an application. Some of those properties include high hardness, high stiffness and strength, corrosion resistance and a wide range of thermal and electrical properties. Ceramics are known for retaining these properties at high temperatures, making them useful in refractory applications. Silicates, oxides, nitrides and carbides are some of the fundamental ceramic materials manufactured today.
Many carbides are important industrial materials. These include calcium carbide, iron carbide, silicon carbide, boron carbide, tungsten carbide, titanium carbide and niobium carbide. These are all synthetic industrial materials. Silicon carbide has received considerable attention for use as a high performance structural material where good strength and toughness retention, oxidation and thermal shock resistance, and high thermal conductivity are demanded at temperatures approaching 1400.degree. C.
Granular silicon carbide is manufactured by the Acheson process, the same as is used for the production of molded graphite parts. In this process, the green solid carbon, usually in the form of large cylinders, are laid out horizontally and packed in granular coke. The mound is then covered with sand (silica). Large water cooled electrodes are fixed at each end of the stack. High current is then passed through the stack which becomes self resistance heated. Temperatures of 2000-3000.degree. C. are generated which graphitizes the solid carbon. At the same time a reaction between the silica and coke packing produces silicon carbide. Traditionally the metallurgical, abrasive and refractory industries are the largest users of silicon carbide. It has also been used for resistance heating elements, in electronic devices and in applications where resistance to nuclear radiation is advantageous.
Traditional ceramic processes for making monolithic products involves the sintering, or densification, of ceramic powders by high temperature heat treatment. This involves both surface and bulk diffusion mechanisms to attain full densification. Sintering results in shrinkage from the green state, making necessary the machining of a hard material when close tolerances are called for. Oxide ceramics are typically processed this way.
Carbides and nitrides do not readily sinter due to limited diffusion in the covalently bonded solids. Many industrial applications make use of a metallic phase which acts as a glue holding together carbide particles (cermets). Other processes for producing monoliths involve high pressure sintering (HIP) or reaction bonding of particles, e.g., reaction bonded silicon nitride.
In some cases ceramic monoliths are manufactured directly from precursors without forming intermediate powders. Chemical vapor deposition is used for producing ceramic coatings. As another example, silicon carbide fibers are produced by pyrolysis of organometallic polymer fiber precursors. The Yajima process entails the use of polysilane polymers which are thermolized to form carbosilane, a polymer with a backbone of mostly alternating silicon and carbon atoms. The carbosilane is drawn into a fiber which is oxidized to promote cross-linking, then heat treated to approximately 1200.degree. C. to form a silicon carbide fiber with a low degree of crystallinity. These fibers (Nicalon.RTM.) are used as reinforcements in high performance ceramic composites.
A relatively new approach for making monolithic ceramics of net shape utilizes carbon foams as a precursor for the reaction conversion to a carbide. Silicon and silicon-refractory metal alloys are used as an infiltrant to form a carbide by reaction at high temperatures. Depending on the void volume of the precursor carbon foam, silicon carbide monoliths with varying degrees of porosity or residual infiltrant have been produced with little or no bulk dimensional change. Porous silicon carbide foam has been considered for high temperature filters and surface combustion plates. It can also be used as a substrate to carry materials such as boron nitride used in semiconductor doping applications.
In summary, many different types of carbon-containing materials are known. The carbonization of wood has been practiced for thousands of years. However, there is still a lack of disclosure of the use of monolithic structures made from carbonized wood. Specifically, no studies have been found relating to methods by which relatively large pieces of wood can be carbonized while retaining their mechanical integrity. Further, no information relating to the production of large crack-free charcoal has been reported. In addition, no study of the reduction in dimensions of wood as a result of carbonization has been done. Measurement of resulting char mechanical properties can not be found in the literature. Furthermore, the successful production of composite materials and ceramics based on carbonized wood having the original structure of the precursor wood has not been achieved. The present invention has been developed in view of the foregoing and other deficiencies of the prior art.